As presently practiced, particle removal is usually achieved by a combination of mechanical and chemical mechanisms. In hard disk manufacturing special dedicated tools are used to scrub or remove particles, often called scrubbers. In many such scrubbers a mechanical particle removal method is used simply with room temperature or sometimes heated DI (deionized) water. Chemicals can be added to enhance the removal efficiency. Cleaning tools differ from scrubbers in that mechanical removal techniques are combined with chemicals instead of simple DI water to remove particles both with a mechanical method and a chemical method combined. Additionally in a cleaning tool, other functions can also be performed in addition to particle removal. For example, the removal of metallic impurities, organic impurities and even wet etching of layers and wet stripping of photoresist can be performed in a cleaning tool in addition to the particle removal function that is the object of this invention.
In a scrubber or cleaning tool for hard disk manufacturing usually a combination of ultrasonics, megasonics and brushes is used to clean the hard disk media, imprint molds and read/write head assembly part. A variety of chemicals is added for removal of particles from hard disk media.
Before the invention of megasonics, ultrasonics was used. Ultrasonics has frequencies in the range of 20 kHz to about 120 kHz.
In the hard disk industry a variety of frequencies are used ranging all the way from 40 kHz up to 1 MHz. Quite often in the hard disk industry a frequency of about 400 kHz is being used.
It is now widely accepted that, in case when there is no mechanical particle method added to the chemistry, and therefore, when particle removal is achieved by chemical contacting only, then the contacting chemistry simply removes particles due to underetching of the particle. The underetching theory goes as follows: a controlled amount of the surface layer is uniformly removed or etched all over the surface of the substrate to be cleaned, usually a hard disk substrate or an imprint mold or a head assembly part. When etching this surface layer, the material underneath the particle is also etched away and this etching releases the particle from the surface. Then, the particle is washed away.
Since the current state of the art for removing particles by chemical means only, relies on undercut etching, and since etching increases with temperature, everyone so far has found that particle removal efficiency increases with temperature.
Megasonics on the other hand relies on cavitation to remove particles and cavitation is not very much temperature dependent. However, since cavitation is very dependent on the dissolved gases, it has been found that megasonics vibration doesn't remove many particles when there are no gases present.
Currently, a cleaning paradox has emerged. Megasonics vibration works well for removing particles and with a very wide temperature range, but the cavitation which the megasonics produces, and which is used to remove particles, also damages the fine patterns and advanced hard disks are starting to use such finely patterned surfaces. Also the imprint molds used for manufacturing hard disks have very fragile patterns. Indeed, the patterns on the hard disks and imprint molds in advanced technologies of hard disk manufacturing are becoming so small that they are very fragile and are very prone to mechanical damage. Now with pattern sizes sometimes as small as 22 nm in 1 dimension, any megasonics power or rather any cavitation will destroy such patterns. Megasonics damage starts to be a problem when the pattern sizes become smaller than 0.3 μm.
Therefore, a new method for removing small particles from the surface of the disks and imprint molds without damaging the fragile structures is necessary. The underetching mechanism, which does not damage the fragile structures, however can also not be used anymore, since the structures are so small, that underetching would remove valuable material from the surface. This is the current cleaning paradox that we are faced with.
Hence the paradox: mechanical particle removal cannot be used anymore for small particle removal, since it also damages the fine patterns, and conventional chemical particle removal by underetching cannot be used anymore, because of loss of surface material which is now a substantial part of the surface.
Even in those cases where the substrate is completely flat and where damage is not a paramount concern, it has been found that for very small particles, the mechanical methods are not effective anymore. Mechanical methods to remove particles, such as, but not limited to, brush scrubbing, spray aerosol bombardment, and ultrasonic and megasonic vibration, are very effective for the large particles, but loose efficiency for the very small particles. Hence, there is a need for an improved chemical method to remove these very small particles even on substrates without patterns. This is the case for current generation hard disks. This is also the case for the read/write head assemblies.
As a summary, there is a great need in the hard disk industry for a solution and a method and an apparatus that can remove small particles from the front side or back side of hard disks and imprint molds without damaging the fine patterns and without substantial underetching of the surface material. There is a general need for an improved chemical method to remove small particles even on substrates without pattern such as conventional hard disks and on read/write head assemblies.
More generally, none of the presently known methods can efficiently remove very small particles, since most of the mechanical techniques loose efficiency for small particles and most of the currently known chemical methods are not very effective for very small particles. The prior art does not provide for an improved chemical method to remove very small particles more efficiently.